1. The Field of the Invention
The present invention is related to ablative insulation for use in rocket motors. More particular, the present invention relates to the use of thermoplastic liquid crystal ablative materials for use in rocket motor insulations.
2. Technical Background
The combustion of a propellant in a rocket motor creates a hostile environment characterized by extremely high temperatures, pressures, and turbulence. The combustion temperature within a typical rocket motor may exceed 5,000.degree. F. Pressure within the motor frequently exceeds 1,500 psi (1.02.times.10.sup.5 g/cm). Gas velocity typically reaches or exceeds Mach 0.2 near the throat of the rocket motor.
It will be appreciated that this is an extremely hostile environment and presents difficult challenges in providing adequate materials which perform well under these conditions. It is particularly difficult to adequately insulate the rocket motor casing and other heat sensitive parts from the extreme environment necessary to produce the necessary thrust. It is well known to those skilled in the art that if insulation within a rocket motor fails, it is probable that the motor as a whole will fail. This may occur, for example, because the propellant burns through the motor case.
The environment is particularly hostile in a solid rocket motor because the combustion gases produced typically contain dispersed liquid droplets of materials such as aluminum oxide. These droplets are believed to produce erosion by blasting the interior of the rocket motor case. While the combustion of a rocket propellant is usually brief, the conditions described above can readily destroy unprotected rocket motor casings, and jeopardize the mission of the motor.
For the reasons stated above, parts of a rocket motor which are exposed to the high temperatures, pressures, and erosive flow conditions generated by the burning propellant are usually protected by a layer of insulation. Various materials, both filled and unfilled, have been tried as insulation. However, it will be appreciated that it is difficult to provide adequate insulation materials because those materials are again subjected to the extreme conditions within the rocket motor.
A number of materials have been tried as rocket motor insulations. Such materials include phenolic resins, epoxy resins, high temperature melamine-formaldehyde coatings, ceramics, polyester resins and the like. These materials, when cured, become rigid structures which are essentially unworkable. Thus, construction of the rocket motor, and processing of these materials, is difficult and tedious. In addition, structures formed from these materials are known to crack or blister when exposed to the rapid temperature and pressure changes occurring when the propellant is burned. Thus, failure of the material is of constant concern.
Another type of well known rocket insulation materials are elastomeric polymers reinforced with asbestos, polybenzimidazole fiber, or polyaramid fiber. These compositions are known as "ablative" insulation because the compositions are partially consumed during combustion, but nevertheless provide protection for the rocket motor. Such materials are generally capable of enduring in a rocket motor long enough to allow complete combustion of the propellant. That is, they erode away sufficiently slowly that adequate protection is provided during the operation of the rocket motor. This rate of material reduction, or "material affected rate" ("MAR"), is expressed in terms of the reduction of the thickness of material per second. The remaining thickness of the material which is used to calculated MAR includes the thickness of the remaining materials, as well as the thickness of the associated char.
As mentioned above, one material which has been widely used as an insulation is asbestos. As would be expected, however, environmental and health concerns have led manufactures to seek acceptable replacements for asbestos in rocket motor case insulation. One alternative elastomeric insulation contains aramid polymer fibers in combination with a powder filler. Another alternative is elastomeric insulation which contains polybenzimidazole polymer fibers in combination with a powder filler. All of these materials, however, are found to have characteristics which limit their use as rocket motor insulation.
To alleviate some of the observed problems with conventional ablative insulation, thermoplastic resins have been used as binders in insulation formulations. These materials undergo endothermic pyrolysis, carrying heat away from the insulation. Thermoplastic resins also have high specific heats, and their pyrolysis products have high specific heats and low molecular weights.
Even in view of the potential advantages of such materials, however, their usefulness is generally considered limited because they have melting points significantly below the temperatures reached inside an operating rocket motor. In addition, conventional thermoplastic resin-based materials readily flow when subject to heat. Therefore, it is conventionally thought that such materials must be combined with thermosetting resins, and be impregnated into a refractory or fiber matrix, to prevent the insulation from melting or running off when exposed to the heat and erosion of a rocket motor.
One example of such a matrix is a resin-impregnated open-celled porous ceramic material. Thermoplastic resins impregnated in the ceramic matrix, however, have been observed to cause the ceramic to crack under thermal shock. Another approach is a composite of asbestos and nylon fiber impregnated with a thermoplastic binder, after which the material is further impregnated with a much larger amount of a thermosetting resin. This process, however, ultimately minimizes the amount of thermoplastic resin actually present in the insulation and again relies on environmentally undesirable asbestos.
When thermoplastic binders have been used outside of the systems described above, a particulate filler is typically added to the insulation. A variety of particulate fillers have been proposed. The most common filler is silica in finely divided form. Another potential filler is powdered carbon (coal). Such a filler is usually required in order to provide an operable thermoplastic insulation material.
It can be readily appreciated, therefore, that ablative thermoplastic elastomers provide potential advantages in the manufacture of rocket motor insulation. These materials are easily processible and moldable. They do not cure by polymerization reactions, as do conventional resin systems. As a result, it is possible to work with the material even after it is in place.
It has also been found that properly formulated thermoplastics can be used as ablative insulation in that they resist the tendency to melt and flow away before their function is completed. Generally such compositions include the thermoplastic resin binder, along with particulate filler and fibrous filler. Even in view of the foregoing, thermoplastic resin binder systems have not been widely adopted because of some of the perceived limitations of such materials.
It would, therefore, be an advancement in the art to provide thermoplastic ablative materials which overcame many of the significant limitations of conventional insulation materials. It would be an advancement in the art to provide thermoplastic ablative insulations which had ablative characteristics superior to those of existing thermoplastic ablative materials. It would be a related advantage in the art to provide such materials which also had good mechanical properties. It would be a further advancement in the art to provide such materials which offered ease of processing and permitted structural components to be fabricated directly from the insulation.
Such compositions and methods are disclosed and claimed herein.